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Old 5th July 2003, 08:27 AM   #1
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Default please give some comment for this preamp sch

power parts
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Old 5th July 2003, 08:29 AM   #2
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amplifier part
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Old 5th July 2003, 08:35 AM   #3
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for all the coupling capacitor, they seems too small? can i use larger value capacitor? for example 4.7U .
will it effect the bandwidth or sound statge?

and for this sch, is there any problems?

i have built one, sound is very good, but not good enough, heee

any thing i can change to make it better? (except changing jesson oil cap / TFK tube~ heee)


thanks
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Old 5th July 2003, 08:56 AM   #4
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Default Looks OK.

The preamp looks OK to me. The unmarked resistor is I guess your grid stopper. I am not quite sure if you need this on a preamp. If you did , remember that this along with the Miller capacitance of the tube will roll off the HF and you don't want this to be in the audio range. I guess this resistor should be 1k or lower. Besides maybe it should be between the input 1Meg resistor and the grid. Maybe you should remove it and see if the system sounds any better.

The rest of the circuit looks OK and why have you circled the capacitors? They have a pretty low Fc . Did you want to change them?

C1/R4 has a Fc less than 1Hz.
C2/R7 has a Fc of about 1.2 Hz
With the 52k feedback resistor this will go up. Worst case it will be just under 10Hz. You could up this to 1uF or a bit more. I would do this only after simulating the circuit as there are so many capacitors in series in the circuit . The resultant phase shift will sometimes cause bass lift at very low frequencies when NFB is present.
C3/V3 has a Fc of less than 1 Hz
C4 with an external 10 K load has a Fc of about 7Hz.
They all seem to be fine. You could change C4 to 4.7 uF if you want. The operating points must be fairly standard. But tubes are sensitive to this and you can playaround with that - very messy thing to do !
I'll see if I can simulate this for you.
Cheers.
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Old 5th July 2003, 09:12 AM   #5
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dear ashok

thanks a lot, for that resistor i am using 100k
i add this resistor is because i wan to lower the input level, since for the VR, i find that it have better performance when it is at 12:00(180 degree). but if i do so, the volume will be too large~
so i add extra resistor to do the attentration.

thannks
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Old 5th July 2003, 09:13 AM   #6
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The cap values look very reasonable, as an example the coupling between V1B and V1A have a -3dB point of ~ 0.72 Hz.

Othervise the amplifier circuit looks very ordinary, one unusual thing is that a dual triode is used as a fullwave rectifier.

I dont really understand why they haven't included the output cathode follower in the feedback loop, it doesn't hurt and will lower the output impedance even more. In this case C3 and R7 can be omitted and the feedback taken from the connection point of C4/R12

For my self I dont like to mix solid state and tube electronics so I use voltage regulators with only tubes but that is just my personal idea.

Regards Hans
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Old 5th July 2003, 09:54 AM   #7
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Hi,

The circuit looks like a phono stage minus the RIAA correction plus some NFB.

No wonder you have way too much gain.

As Hans suggested, they could've included the CF into that loop to lower Zo and increase linearity.

Or you could try out a higher value for the NFB resistor to reduce gain.

They chose a 12AX7 and a 12AU7 for the line stage proper....but than go against the grain of availability and pick a 5687 for full wave rectification and as a series pass device...odd.
And since we're using valves there already, why not go all the way and use a neon as voltage reference instead of the noisy zeners? And a tubed error amp of course...

Especially when keeping in mind that a line stage could've been built with a single 5687 in the first place, the whole thing looks like something thrown together by someone not knowing what to do...no offense.

I would change the values of the coupling caps either, it would likely only make matters worse.
You could up their quality, if you think the circuit is worth it.

If it has a source selector, you can consider hooking it up so that you can only use the CF and/or if you need gain use the second half of the 12AX7 only ( that will invert absolute polarity).
After that, if you notice significant sonic improvement you can consider replacing the volpot for an attenuator.

Cheers,
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Old 5th July 2003, 01:30 PM   #8
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Default Some results.

Hi Takashi,
I ran some simulation using BeigeBag software.

As I expected there is a big bass lift at very low frequencies. It does not show up in the output because of the losses in the following capacitors. The feedback circuit tries to overcome the loss due to the capacitors and boosts the bass within the circuit. At low levels this may not be too much of a problem but a high signal levels the stages would be driving much harder than required. The signal is measured at the anode of V2.
Look at the graphs attached.
The coupling capacitor C2 is 0.33uF and with that value the gain at 2Hz is +12.2db higher than the gain at 1kHz. With C2=1uF the 2Hz gain is +4.4db and with C2=4.7uF the gain is only +0.38db higher. If I were you I would use 4.7uF or 2.2uF at the C2 position.

I also dumped the Cathode follower (V3) circuit to see how much the output impedance was . I used a 10 K ohm load. The 680K was made 1Meg ohm and C3 onwards everything was deleted. C2 was made 4.7uF.
The distortion with a 10K ohm load was 0.15% at 1Khz and about 5.5 Volts output. Output impedance was 702 ohms. At 20Khz the output impedance was 754 ohms and at 20Hz it was 702 Ohms.

Not bad at all. You could use it that way also.
Note that the distortion with the Cathode follower with about the same output is 0.04% because the load on V2 is now very high.
The gain of the circuit is about +13.9db ( x4.95).
I used the 100k resistor at the input like you did. It should not reduce the gain too much. Remember that the stray capacitances at the input are not included and so the HF will roll off faster than the analysis suggests. Also any series resistor at the input degrades the noise performance of the circuit. better to use a resistor at the top of the volume pot. Stray capacitance will still roll off HF .You will have to check this out.
Cheers.
There is a problem with the attachment. I will attach it in the next post.
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Old 5th July 2003, 01:44 PM   #9
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Default The doc file with the curves

Here is the zipped doc file.
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Old 5th July 2003, 05:49 PM   #10
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thanks a lot

by conculsion, i need to
1) remove the un-labled resistor to have a better HF response.
2) increase the value of C2 to 4.7u and i can remove C3 and R7
3) get the feedback loop from V3 at the point between C4 and
R12

but for 3), the 52k FB resistor no problem? lower the resistor will have a deeper feedback? and what is the relation between deepness of FB and the sound(distortion)

thanks once more~
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